1. Field of the Invention
The present invention relates to an impact absorbing member for vehicle, for use in a vehicle for absorbing an impact applied to the vehicle. More specifically, the present invention related to a technique, applied to the case where a vehicle is subjected to an impact load obliquely from the front or back thereof, so as to prevent an impact absorbing member from falling down laterally and losing the impact-absorbing properties.
2. Description of the Related Art
As an impact absorbing member for vehicle, the following one is known. This impact absorbing member for vehicle has a tubular shape, and is provided with a concave groove at a portion of its side wall. The concave groove concaves inward in a direction orthogonal to an axial direction of the tubular shape (the direction perpendicular to the axis), and extends parallel to the axial direction. This impact absorbing member for vehicle is disposed between a vehicle body side member and a bumper member in such a manner that the axial direction thereof is coincident with an fore and aft direction of a vehicle. When subjected to a compressive load, the impact absorbing member collapses like an accordion in the axial direction, thereby absorbing the impact energy (See Patent Document 1: WO 2005/010398).
FIGS. 9A and 9B explain one example of such an impact absorbing member for vehicle. FIG. 9A is a schematic plan view showing the vicinity of a bumper beam 10 on the vehicle front side, as viewed from the top of a vehicle. Right-hand and left-hand side members 12R and 12L are provided with, at the front ends thereof, crash boxes 14R and 14L, respectively, as impact absorbing members. The bumper beam 10 is fixed on the right-hand and left-hand crash boxes 14R and 14L at such ends.
FIG. 9B shows section IXA-IXA of FIG. 9A, i.e., the section near the right-hand mounting position. The crash box 14R comprises a body portion 20 having a tubular shape and a pair of mounting plates 22 and 24 integrally weld-fixed to both axial ends of the body portion 20, respectively. Via these mounting plates 22 and 24, the crash box 14R is fixed to the side member 12R and the bumper beam 10 with a non-illustrated bolt or the like.
FIGS. 10A and 10B specifically explain an example of the body portion 20 of the above crash box 14R. FIG. 10A is a perspective view, and FIG. 10B is a front view. A section perpendicular to the axial direction of the tubular body portion 20 (see FIG. 10B) has a basic shape of an elongated shape having a pair of parallel longer sides obtained by linear interpolation (connection) between two adjoining sides (a vertically elongated octagonal shape, in case of FIG. 10B). A pair of wider side wall portions 30 giving the longer sides of the basic shape is each provided with a concave groove 32 at middle portion in the width direction thereof, i.e., the vertical direction in FIG. 10A and FIG. 10B (the center portion in FIG. 10B). The concave groove 32 concaves inward and extends parallel to the axial direction.
The body portion 20 is disposed between the side member 12R and the bumper beam 10 in such a manner that the pair of the wider side wall portions 30 are located in the width direction of the vehicle. The body portion 20 may be integrally formed by hydrostatic forming using a tubular pipe material, i.e., a single member or the like. However, the body portion 20 shown in FIGS. 10A and 10B is formed of a pair of pressed plate materials 26 and 28. This is a polygonal tube having a vertically elongated shape, obtained by integrally weld-fixing the pair of pressed plate materials 26 and 28 having been bent into an M shape, in a state that both side portions of one pressed plate material are superposed on both side portions of the other pressed plate material.
When such a crash box 14R is subjected to an impact applied from the front of the vehicle and receives a compressive load F, the body portion 20 collapses like an accordion as shown in FIG. 9C. The deformation at this time absorbs the impact energy, thus relieving the impact applied to the side member 12R and like structural members of the vehicle. The accordion-like collapse is a phenomenon caused by continuous buckling of the body portion 20 at a large number of portions axially spaced from one another (L-shaped folds in FIG. 9C). Buckling usually starts from side of the bumper beam 10, i.e. the input side, and progresses toward the vehicle body side with time.
The bumper beam 10 is symmetrical, and has the same structure at the left-hand mounting position. Further, this bumper beam 10 functions as a bumper reinforcement (reinforcing member) and a mounting member, and a bumper body 16 made of synthetic resin or the like can be integrally mounted thereon. The bumper beam 10 corresponds to the bumper member, and side members 12R and 12L correspond to the vehicle body side members, of the present invention, respectively.
By the way, in recent years, for crash tests for evaluation of damages on vehicles, a test method using an oblique barrier assuming collisions into vehicles from oblique directions have been adopted. According to this test method, as shown in FIG. 11A, a vehicle is offset-crashed into a rigid barrier 42 having a crash surface 40 at a predetermined angle θ1 (e.g., 10°), at a predetermined vehicle speed V1 (e.g., 15 km/h). Further, as shown in FIG. 11B, a crash cart 46 provided with a barrier 44 at its front end is crashed into a corner of the vehicle from a direction at a predetermined angle θ2 (e.g., 10°) at a predetermined speed V2 (e.g., 15 km/h).
According to such a test method, for example, as shown in FIG. 12, when a load F acts obliquely to the axial direction of the crash box 14R and generates a moment load M, the crash box 14R often falls down toward the vehicle inner side (the left in FIG. 12). As a result, original impact-energy absorbing properties of the crash box 14R may be impaired. FIG. 12 is a plan view showing the right half of the bumper beam 10.
FIGS. 13A to 13F show a result of FEM simulation of collapse process of the crash box 14R upon a crash test conducted using a 40% offset rigid barrier shown in FIG. 11A under conditions of angle θ1=10°, and vehicle speed V1=15 km/h. The crash box 14R falls down laterally in the phase shown in FIG. 13E.
FIG. 5A is a graph showing a relation between compression stroke and load. FIG. 5B is a graph showing a relation between compression stroke and absorbed energy. In FIG. 5A and FIG. 5B, a dashed line shows the calculated results of the load and absorbed energy during the crash test. As is obvious from these data, the load starts falling approximately when the compression stroke exceeds ST1. This suggests that it was around this point on the graph when the crash box fell down laterally.
Against these problems, measures as shown in FIG. 14A and FIG. 14B can be conceived, for example, although this has been unknown. According to the measure shown in FIG. 14A, a crash box 14R is rotated at 90° around an axis thereof to have a horizontally elongated shape, so as to be prevented from falling down laterally. This structure requires no great modification to the crash box 14R itself, and can thus be easily applied. However, due to the increased dimension in the width direction of the vehicle, the mountability of the crash box 14R on a vehicle is degraded. As a result, a substantial modification in design may be required, such as an increase in the width dimension of the side member 12R, etc.
According to the measure shown in FIG. 14B, an auxiliary member 48 for canceling the moment load M explained in FIG. 12 is disposed parallel to the body portion 20 and between the mounting plates 22 and 24. This causes problems such as an increase in the number of components followed by increases in cost and weight, the reduction of productivity, a mountability deterioration of the entire components on the vehicle, and the like.